Imaging sensors form an image of a scene onto an imaging detector. The detector produces a responsive signal output, which is typically digitized and analyzed in digital form. Imaging sensors are widely used in military applications and increasingly in civilian applications.
The signal output of the imaging detector is automatically analyzed by a signal-processing computer for the presence of features of interest in the field of view of the sensor. The automated analysis of such images requires extensive computational power, because the field of view of the sensor may also include background clutter and other features, some of which may be similar to the features of interest. In military applications, the features of interest may be camouflaged to reduce their contrast with the background and to increase their similarity to other features in the field of view. Similar-appearing decoys may also be present.
One technique for improving the recognition of features of interest and reducing the computational power required is to perform analog image processing based upon image characteristics that aid in distinguishing features of interest. One specific type of analog image processing is a polarization analysis. Some features of interest may be identified from other features in the field of view by forming images of different polarization states of the features and then analyzing the differently polarized images. For example, many artificial features such as man-made objects exhibit differently polarized reflected light images, while natural features do not exhibit such differently polarized reflected light images.
The available polarizing imaging sensors utilize polarizers that form the differently polarized images sequentially. For example, a p-polarized image may be formed, and then the s-polarized image is formed shortly thereafter. This sequential polarization is necessary because the reflected light images in the scene are polychromatic. The available polarimeters that simultaneously form differently polarized images produce a chromatically aberrated image of each polarization state that is not useful for subsequent comparative analysis with the image of the other polarization state. The problem with using sequentially polarized images is that the features in the images may change position or shape slightly in the time required to form the sequential images. That is, the features may move relative to each other in the field of view or change aspect ratio even in the short time required to form the two differently polarized images. This relative movement in the p-polarized image and the s-polarized image greatly complicates or makes impossible the analysis of the digitized images by the imaging detector and the associated electronics. Another approach is to use two different detectors, with each detector simultaneously sensing a differently polarized image. This approach is excessively costly and adds too much weight and size for many applications.
There is therefore a need for an improved approach for producing and analyzing polarized images in an imaging polarizing sensor. The present invention fulfills this need, and further provides related advantages.